Idea: AssertJ 4 to change behaviour of instanceof checks

[ascopes]

At the moment, if I am writing a test that checks the type of an object, I still have to manually cast after the check to downcast to the type I want to use.

For example... suppose I am testing the following code:

interface Validator {
  void validate(Object obj);
}

interface PluggableValidator extends Validator {
  void loadPlugin(Plugin plugin);

  Collection<Plugin> getPlugins();
}

...and my test is this:

@Test 
void pluginsHaveTheExpectedValue() {
  Validator validator = factory.newValidator();

  assertThat(validator)
      .isInstanceOf(PluggableValidator.class)
      .satisfies(v -> assertThat(((PluggableValidator) v).getPlugins())
          .contains(...));
}

Right now, I have to still explicitly cast after the instance check. What I'd probably expect though is for methods like .isInstanceOf(Class<T>) to return a new ObjectAssert<T> for generic object assertions, such that the following is valid:

@Test 
void pluginsHaveTheExpectedValue() {
  Validator validator = factory.newValidator();

  assertThat(validator)
      .isInstanceOf(PluggableValidator.class)
      .satisfies(v -> assertThat(v.getPlugins())
          .contains(...));
}

I'm not sure if this is likely to introduce other implications due to the restrictiveness of the compiler when handling generics. Aware that varying the returned assertion type is not going to be possible with this since you cannot overload generic signatures at compile time.

[joel-costigliola]

+1 if we can do it smoothly, which is always a bit of challenge with generics

[scordio]

Is asInstanceOf with InstanceOfAssertFactories.type(Class) not an option for your use case?

Happy to look at your real project if it's publicly available on GitHub.

Following your example, this should be possible:

@Test 
void pluginsHaveTheExpectedValue() {
  Validator validator = factory.newValidator();

  assertThat(validator)
      .asInstanceof(type(PluggableValidator.class))
      .satisfies(v -> assertThat(v.getPlugins())
          .contains(...));
}

[ascopes]

Ahhh I wasn't aware of this! That is exactly what I need.

Thanks!

[scordio]

Or even:

@Test 
void pluginsHaveTheExpectedValue() {
  Validator validator = factory.newValidator();

  assertThat(validator)
      .asInstanceof(type(PluggableValidator.class))
      .extracting(PluggableValidator::getPlugins, as(LIST))
      .contains(...));
}

with as and LIST.

[scordio]

I'm closing the discussion given that asInstanceOf exists, but let us know if you need further help or if something is missing!